1. Field of the Invention
This invention relates to a method and apparatus for generating images from physiological data acquired at multiple points in a volume. In particular, the invention relates to a method of generating images, the method accommodating the physical constraints of conventional computer hardware so as to increase the speed with which diagnostic images may be transformed and displayed.
2. Background Art
Advances in technology have provided the medical practitioner with a variety of medical imaging systems. One important class of medical imaging systems works by radiating energy through a region of interest in the body, either from an external source, such as x-rays or an RF field, or from an internal source, such as an injected radioisotope. The interaction of the radiated energy with the body is measured at a variety of small volume elements ("voxels") within the body. The spatial coordinates of each of these voxels are identified and a map of the body within that region of interest is developed.
Two important types of medical imaging systems which provide such voxel data are Magnetic Resonance Imaging ("MRI") and X-ray Computed Tomography ("CT")
a. Magnetic Resonance Imaging
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species, i.e. the gyromagnetic constant .gamma. of the nucleus. Nuclei which exhibit this phenomenon are termed "spins".
When a substance such as human tissue is subjected to a uniform magnetic field, the individual magnetic moments of the spins in the tissue precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another.
If the tissue is subjected to an oscillating radio frequency magnetic field which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, rotating in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence the magnitude of the net transverse magnetic moment M.sub.t, depends primarily on the length of time and the magnitude of the applied excitation field.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal is terminated. In simple systems, the excited spins induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude is determined by the magnitude of the transverse magnetic moment. The amplitude of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.O e.sup.-t/T* 2 (1)
The decay constant 1/T.sup.*.sub.2) depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase, after removal of the excitation signal, in a perfectly homogeneous field.
Another important factor which contributes to the amplitude of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. It describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques is known which involves the application of one or more RF excitation pulses of varying magnitude, duration, and direction. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.O, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
Typically, the volume which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles. The resulting set of received NMR signals are digitized and processed to reconstruct data indicating the physical properties of specific voxels within the imaged volume. The voxel data is stored in an array in memory so that the spatial coordinates of each voxel may be identified.
b. Computed X-Ray Tomography
In a computed tomography system, an x-ray source is collimated to form a fan beam with a defined fan beam angle. The fan beam is oriented to lie within the x-y plane of a Cartesian coordinate system, termed the "gantry plane", and is transmitted through an imaged object, such as human tissue, to an x-ray detector array oriented within the gantry plane.
The detector array is comprised of a set of detector elements each of which measures the intensity of transmitted radiation along a different ray projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the tissue.
The x-ray source and detector array are rotated on a gantry within the gantry plane and around a center of rotation within the tissue so that the angle at which the fan beam axis intersects the tissue may be changed. At each gantry angle, a projection is acquired, comprised of the intensity signals from each detector element. The gantry is then rotated to a new angle and the process is repeated to collect a number of projections along a number of gantry angles to form a tomographic projection set.
The acquired tomographic projection sets are typically stored in numerical form for later computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. The reconstruction process converts the data of the rays, which represents total attenuation along the lines of the rays, to voxel data which represents the incremental x-ray attenuation provided by a voxel within the scanned area.
A typical computed tomographic study involves the acquisition of a series of "slices" of the imaged tissue, each slice parallel to the gantry plane and having a slice thickness dictated by the width of the detector array, the size of the focal spot, the collimation and the geometry of the system. Each successive slice is displaced incrementally along a z-axis, perpendicular to the x and y axes, so as to provide a third spatial dimension of information. After reconstruction, voxel data over a volume of the patient is obtained.
c. The Display of Voxel Data
As noted above, each of these imaging systems employs energy radiated through a region of interest of the body and detects the interaction of the energy with the body to acquire data at a plurality of voxels. The type of radiated energy is different for different imaging systems. The radiated energy is x-rays in the case of computed tomography (CT) and a radio frequency magnetic field in the case of nuclear magnetic resonance imaging (MRI). The energy is sound waves, in the case of ultrasound and high energy particles from an injected radioisotope, in the case of nuclear medicine or PET scanning. In each case voxel data is obtained over a volume.
The processing and display of the voxel data produced by the above systems present two problems. The first problem is simply the large amount of data that is acquired: a typical study may generate data for over 4,000,000 voxels. Efficient processing of this data is essential if the technology is to be practically useable.
The second problem is the need to present the voxel data in a meaningful way. The data is unintelligible when represented as numerals alone, so typically, each voxel value is projected to a brightness value of a picture element("pixel") on an image plane. The pixels of the image plane together produce a picture-like image. The image may be more easily understood to a human operator.
Projecting the voxels of the three dimensional volume to the pixels of a two dimensional picture requires discarding some voxel information. Ideally, however, an appropriately selected image plane will capture most of the diagnostically significant information. Alternatively, several image planes may be generated, and viewed, sequentially. In this latter case, just as one may comprehend the three dimensions of a statue by viewing it from various angles, the multiple projections allow one to gain a more complete understanding of the voxel data.
In both cases, it is critical that the orientation of the image plane be freely selected, easily adjusted and that new images be rapidly generated. This allows the ideal image plane to be determined on an interactive basis, or permits the quick shifting between different image planes necessary to visualize the three dimensions of the data.
For the projection of high resolution images, it is known to employ a ray tracing technique in which a series of rays are generated normal to the imaging plane and spaced to correspond with the spacing of each pixel in the desired image. For each ray, the ray's path through the imaging space is calculated and the voxel having spatial locations closest to the ray read from the computer memory. This voxel data is used to generate interpolated values at points lying on each ray. These interpolated values are combined to generate the value for the particular pixel associated with that ray. This method requires the reading of large amounts of voxel data at random memory addresses and an unwieldy number of interpolations and is therefore unacceptably slow when used to process the large amounts of voxel data associated with typical medical imaging systems. Ideally, one should be able to rotate and project medical imaging voxel data on a near "real-time" basis much as one might rotate a physical object within one's hands.